Funny you should ask that question-I haev a reasonably large Evap Dooler and Box fan in my Shop, and it just wasn't cutting it, so last weekend I went to Lowe's and they have a portable 10,000 btu A/C unit-I bought it, had all three running last Saturday and it could barely keep up (it was 100 outside)-go as big as you can-

I have a 21,000 BTU window unit in my 24X28 Attached garage and I rarely turn it past medium on the dial. It was in my FIL's house for I don't know how many years before I got it 15 years ago when they moved. I also have 3-1/2" insulation on all walls & ceiling except the wall where the OH door is but I also have an insulated steel 16X7 door. I keep waiting for it not to work someday but that day hasn't arrived yet. By the way, here in SW Kansas today it hit 99 and the garage/shop was a comfortable 78.

I use a 8000 btu to cool my 40x60x12. But it has 10" insulation in the walls and 22" in the ceiling. It hasn't been over 75 in the shop yet this year so I haven't fired it up yet. Maybe this weekend . It is supposed to be over a 100 for the next few days. Anyway, how big depends on how well your shop is insulated and how well the shop is sealed. Air leaks will raise the threshold a lot. If you have 4" inthe walls and 10 in the ceiling, you should be able to get along with 20,000 btu.

I've got a heat pump on my 24x30 shop. I'll look when I get home, can't remember what size. I hosed out the shop Saturday afternoon, it was over 100........shut the door, turned the thermostat to 75, and it got downright COLD in there pretty quick. I like having the thermostat to take of the temp. In the summer, I leave it at 85 when I'm not in it, in the winter I leave it at 50 when I'm not in it. I did noticve a couple of weeks ago that I may have to lower the temp a little in the summer, the grease in my grease gun had started to leak from being a little too hot. Sure makes for nice working conditions year round though.

The old rule of thumb used around here for home A/C is one ton (12,000BTU) for every 600 square feet of living area. That assumes typical home construction, stud walls with sheetrock on the inside & wood outside, no significant insulation, decent weatherstripping, and normal occasional door opening (YER LETTIN ALL THE COLD AIR OUT DAMMIT!!!). Add more if it's a single-thickness wall (especially metal), or leaky, or you leave the door open a lot, etc. Lotsa light bulbs and power tools also increase the load.

Don't forget the dehumidification factor. A too-big unit will cool, but it won't run long enough to get the water out of the air, leaving you with a clammy feeling.

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